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An international group of physicists has first discovered rare solar neutrinos that are the result of one of the types of thermonuclear reactions that convert hydrogen to helium.
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The particles were captured by the Borexino observatory at the National Laboratories of the Gran Sasso, in Italy, and their in-depth study could help to solve the enigma of the metallicity of the Sun.
He published finding for the Neutrino 2020 virtual conference supports the theory that solar neutrinos should arise from proton cycle and CNO cycle (carbon, nitrogen-oxygen). In the first case, four hydrogen protons fuse to form helium, and this cycle represents almost all of the energy released by the Sun (more than 99%). The second scenario, which includes massive stars, involves the nuclei of carbon, nitrogen, and oxygen atoms in a thermonuclear reaction.
So far, physicists have only found neutrinos that form as a by-product of the proton cycle while other particles remain elusive.
The neutrino from the CNO cycle has been captured thanks to the installation of a kind of steel sphere filled with 280 tons of liquid scintillation, a substance capable of emitting light when struck by particles. Thus, the flashes of light capture several thousand photomultipliers mounted on the inner surface of the sphere. Although tens of billions of neutrinos traverse a square centimeter every second, it only manages to record a few dozen neutrino-liquid interaction events within the sphere each day.
The intensity of the neutrinos flux and their energy can be used to determine the nuclear reaction that generated them. The main problem was to distinguish neutrinos that arose in the CNO cycle from particles that are generated by traces of decaying bismuth-210 isotopes with the formation of polonium-210. Bismuth, in turn, arises from the decomposition of lead isotopes contained on the interior surface of the ship. Meanwhile, the polonium leaked into the liquid and spread out there due to thermal mixing, making it difficult to quantify the contribution of bismuth to the neutrino signal.
Scientists also installed special thermal insulation screens to prevent the scintillator from convulsing inside the sphere, and they supervised the stabilization of the liquid for several years. Only after this long procedure was it possible to separate the neutrino signals, the sources of which were both bismuth and the CNO cycle.
The results of the experiment concluded that 700 million CNO cycles pass through one square centimeter of the Earth’s surface every second. The discovery of these neutrinos will help scientists accurately determine the metallicity of the Sun’s surface, that is, the content of elements heavier than helium.
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